With the demand for higher levels of integration of semiconductor chips, such as silicon semiconductor chips, there is a need for greater density and a demand to reduce the amount of silicon used for each of the circuits. This is especially the case with a SRAM, either on a microprocessor integrated circuit chip, of which a large portion of the silicon is a SRAM, or as a separate chip. For increased performance of future microprocessor, the storage capacity of the SRAM must increase thereby requiring a larger portion of the silicon of the microprocessor as the SRAM or a larger separate SRAM chip.
A 1-bit storage cell in a SRAM consists of a simple latch circuit with two stable operating points or nodes. Depending on the preserved state of a two-invertor latch circuit, the bit of data being held in the cell will be interpreted either as a logic "0" or as a logic "1". To access the data in the cell via a bit line, a switch is controlled by a corresponding word line carrying a row address selection signal. Two complementary access switches are used to connect the 1-bit SRAM cell to the complementary bit lines. A field effect transistor (FET) SRAM cell consists of two cross-coupled invertors and two access transistors. The load devices may be polysilicon resistors, depletion-type N-type FETs, or P-type FETs depending on the type of SRAM cell. Pass gates acting as data access switches are enhancement-type N-type FETs. Of these load devices, the use of resistive-load invertors with polysilicon resistors in the latch structure results in a significantly more compact cell size, compared with the other alternative described, because it reduces the cell size to four transistors in contrast to six transistors of the other alternative. However, creating a polysilicon load resistor is quite difficult and expensive from manufacturing standpoint in that, not only is a critical mask required for the polysilicon load resistor, but another critical mask is required for the contact to the load resistor, conventionally called a quasi-buried contact (QBC). In addition, care must be taken to isolate the polysilicon load resistor. Although a load resistor takes less layout space than a FET, the resistor still takes up a significant amount of space. Further, special implants and other special processing is usually required in fabricating the polysilicon load resistor.